1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Description of the Related Art
In laser printers, which are image forming apparatuses, a photoconductor, which is an image carrier, is scanned in a horizontal scanning direction (i.e., main scanning direction) by optical beams (laser beams) deflected by a deflector to write an image on the surface of the photoconductor. The deflector can be, for example, a rotating polygon mirror.
Optical beams are deflected at an isometric velocity in the horizontal scanning direction by the deflector, and the optical beams are corrected from deflection at the isometric velocity to deflection at a uniform velocity by a fθ lens.
The fθ lenses can be made from different material including plastic. However, if an fθ lens made of plastic is used, its shape and/or refractive index can change with the surrounding temperature. If the shape or the refractive index changes, a scanning position on the photoconductor deviates, which results in a magnification error in the horizontal scanning direction. The magnification error leads to a degraded image. The refractive index also changes with the wavelength of the laser beam.
Various technologies have been proposed to correct the magnification error. In one approach, laser beams scanned in the horizontal scanning direction are respectively detected by laser beam detectors provided at two positions in the horizontal scanning direction, the time difference between detections of the laser beams in the two detectors is measured, and the magnification in the horizontal scanning direction is corrected based on this time difference.
Laser printers are disclosed in Japanese Patent Application Laid-open No. 2003-279873. In one of these laser printers, a scanning target surface is scanned in the horizontal scanning direction by beams of light deflected by a deflector, the beams of light are respectively detected on a write start position side and a write end position side, to correct the phase data based on the fluctuation amount of time required for scanning between the two positions, and the phase of respective signals of an image clock that performs image formation based on the phase data is shifted (phase modulation), thereby correcting the magnification of the image in the horizontal scanning direction on the image carrier.
In other laser printer disclosed in Japanese Patent Application Laid-open No. 2003-279873, the whole misregistration of dots in the horizontal scanning direction is shifted by changing the frequency of the image clock (frequency modulation), to correct the magnification of the image in the horizontal scanning direction on the image carrier.
The laser printer that corrects the magnification of the image by phase modulation in which the phase of the image clock signal is shifted can change the correction amount in a short period of time, and hence, correction can be performed in between sheets of paper (at the timing when image formation is not performed), when images are continuously formed. However, since image degradation occurs more or less as compared with the magnification correction of the image by frequency modulation, there is a problem in that when the phase shift amount of the image clock signal increases, degradation in the formed image increases.
On the other hand, when magnification correction of the image is performed by frequency modulation in which the whole misregistration amount of dots is shifted, a better image can be obtained as compared to the one obtained by phase modulation. In the case of frequency modulation, however, a phase-locked loop (PLL) circuit is normally used for generating a pixel clock for modulating the laser beams corresponding to an image signal. The PLL circuit includes a voltage-controlled oscillator that changes the frequency according to the applied voltage, and it is necessary to stop the printing operation until the PLL oscillating frequency is stabilized after having started a change in the oscillating frequency of the PLL.
That is, in the case of a method of correcting the frequency of the image signal, for example, number of prints and time are counted, and magnification correction of the image by frequency modulation is performed at an interval of certain time that is considered not to cause a side effect such as image degradation. In this case, however, image forming operation is suspended in order to correct the frequency of the image signal. As a result, the number of suspensions increases, and the overall print speed (number of image formations per unit time) as an image forming apparatus decreases drastically.
Further, when magnification correction of an image is performed by frequency modulation, if the timing for performing the frequency correction is previously set, then even when the magnification error in the horizontal scanning direction increases in the period after frequency correction of the image signal until the next frequency correction is performed, frequency correction is not performed until the timing for the next frequency correction, and hence, a degraded image is formed during this time.